Towed low-frequency underwater detection system

ABSTRACT

The invention relates to low-frequency underwater detection systems comprising a towed linear antenna ( 12, 13 ). 
     It consists in producing the transducers of the transmission antenna ( 12 ) in the form of flextensional arrays of cylindrical type ( 20 ) and in forming directional transmission channels covering all of space. 
     It makes it possible to lighten the assembly and to facilitate implementation at sea, which becomes able to be automated.

The present invention concerns underwater detection systems which use atleast one linear reception antenna towed by a surface vessel or by asubmarine. More particularly, it relates to systems for activatinglinear antennas that are towed from a surface vessel and that can bewound up on winches.

In the known systems, the surface vessel tows a fish which comprises theacoustic transmitter composed of several low-frequency transducers andto which the linear reception antenna is hooked up. The description ofsuch a system will be found for example in French Patent No. 95 07228filed on Jun. 16, 1995 by the company Thomson-CSF, published on Oct. 31,1996 under No. 2 735 645 and granted on Jul. 30, 1997. These systemsdemand considerable means as far as placement in the water and recoveryon board the boat are concerned. As regards hardware means, the boatmust have at least one crane associated with a winch, and in terms ofhuman means, experience shows that at least 3 people are required inorder to perform the maneuvers under conditions of safety whichnevertheless remain mediocre, or even poor in heavy seas.

In order to fix matters, a fish such as that described in the aforesaidpatent weighs about 2 tonnes in air. Thus, in particular duringrecovery, it is necessary to raise the fish after winding up the heavycable on a winch with the linear antenna hooked behind, and then todisconnect the antenna and wind it up on a second winch.

According to the prior art, for example described in French Patent No.94 15109 filed on Dec. 15, 1994 by the Délégation Générale à l'Armementand published on Jun. 21, 1996 under No. 2 728 425, the acoustictransmitter is formed of a linear antenna comprising electroacoustictransducers of Tonpilz type with 2 horns. According to an embodimentdescribed on page 19 and FIG. 6, the transmission antenna is followed byone or more linear reception antennas, one at least having ambiguityremoval. Even though this system resolves the handling problem mentionedpreviously and makes it possible to obtain a lighter anti-submarinewarfare system than the known systems, it has the drawback of beingdirectional in transmission, this being quite in accordance with the aimof the invention described in this patent, which is to go from a knownvolumic transmission system to a linear system that is very directionalin a horizontal plane. Hence, the use of transducers of Tonpilz typewhose emissive faces are situated on the axis of the antenna does notallow the formation of channels in directions inclined with respect tothis axis. This stems from the acoustic interactions between thetransducers that mutually insonify one another. Moreover the diameter ofthe antenna is large, of the order of 20 cm, this posing enormousproblems with regard to winding it up on a winch.

Under these conditions, the operational benefit of this device is muchreduced.

To alleviate these drawbacks, the invention proposes the use of a lineartransmission antenna composed of flextensional transducers ofcylindrical type and combined with a system for forming channels ontransmission covering all of space.

According to another characteristic of the invention, 3 distinct modesof transmission are used, one directional, the other sectorial, and thethird of the known RDT type.

According to another characteristic of the invention, the RDTtransmission mode is a double-beam mode.

According to another characteristic of the invention, a receptionantenna comprising trios of hydrophones integrated in a rigid mannerinto a single linear antenna is associated with the transmissionantenna.

Finally, according to another characteristic of the invention, thediameters of the transmission and reception antennas are equal.

Other features and advantages will become clearly apparent in thefollowing description, given by way of non-limiting example with regardto the appended figures which represent:

FIG. 1, a diagrammatic view of the entire system;

FIG. 2, a perspective view of two transmission assemblies;

FIG. 3, a sectional view of a transducer 20 of FIG. 2;

FIG. 4, a picture of the directional and sectorial transmission modes;

FIG. 5, a phase correction law for the transmission signals;

FIG. 6, a picture of the RDT transmission mode;

FIG. 7, a chart of the transmission times in this RDT mode;

FIG. 8, a perspective view of an assembly of assembled receptionmodules; and

FIG. 9, an exploded perspective view one of these modules.

In FIG. 1 which represents a diagram of the entire system, a surfacevessel 10 tows a transmission antenna 12 and a reception antenna 13 byway of a heavy cable 11. In a known manner, damping modules 112 and 113reduce the vibrations caused by the towing as well as by a tail cable114.

Represented in FIG. 2 is an exemplary embodiment of two transmissionassemblies which will constitute a transmission antenna after they havebeen sheathed. Each assembly is formed of a flextensional transducer 20and of a cylindrical container 21 making it possible to adjust thebuoyancy of the assembly. The transducer is of the split monocoqueflextensional type as described for example in French Patent No. 9510534 filed on Sep. 8, 1995 by the Applicant, published on Jan. 20, 1997under No. 2 738 704 and granted on Oct. 7, 1997. Rings 200 furnishedwith three 120° lugs 201 allow the holding and the centering of thetransducers in the sheath. At the head of the antenna, a module (notrepresented) contains the electrical transformers making it possible tostep up the voltage and to tune them to the transmission frequency. Theyare linked to the two control wires 30 of the piezoelectric motors ofthe transducers represented as a longitudinal section in FIG. 3.

Each transducer being essentially capacitive of value C, tuning iscarried out on the basis of the inductance L of the transformer byapplying the formula ${\omega = \frac{1}{\sqrt{LC}}},$ω being the angular frequency corresponding to the transmissionfrequency.

This module receives the signals from the surface vessel by way of theelectric suspension cable 11.

According to the invention, channels are formed on transmission in allof space by using in a known manner signals obtained from a digitalprocessing assembly situated on board the boat.

Three modes of transmission are used for this purpose:

-   -   a mode known by the name RDT standing for “Rotary Directional        Transmitter”, over 360°,    -   a sectorial mode,    -   a directional mode.

With each mode of transmission is associated a digital channel formingby delay, also known in the art as “Inverse Beam Forming”.

In conventional manner, the transmissions are formed of CW or hyperbolicFM pulses, or a combination of the two, or else BPSK (Binary PhaseShifting Key) over variable durations. To form a channel, the signalsgenerated from frequency synthesizers and digitized are delayed with aspecified delay value for each flextensional transducer, then amplifiedso as to be sent to the transmission antenna.

FIG. 4 diagrammatically represents the radiation patterns obtained indirectional mode and sectorial mode. In directional mode, the beam is asnarrow as possible given the resolution of the antenna and to each pulsetransmitted there corresponds a different direction.

It is recalled that for a linear antenna, the radiation pattern exhibitsa volume with symmetry of revolution about the axis of the antennaexcept for the 2 right/left directions on the axis of the antenna,referred to as “end-fire”. It is also recalled that the lobe widths arevariable, from the narrowest on the side perpendicularly to the axis ofthe antenna, referred to as “broad-side”, to the widest “end-fire”.

The channel formings are conventionally obtained by delays or phaseshifts introduced onto the signal from each flextensional transducer,this signal being provided by frequency synthesizers depending on thetype of pulse transmitted. The delays are computed digitally on thebasis of commercial electronic cards.

In a known manner, the sectorial mode is obtained by widening the mainlobe of the transmission beam by programming a delay or phase lawadapted to the signals from the transducers, for example a law ofquadratic type.

According to an exemplary embodiment, the transmission antenna comprises16 transducers and the phase correction law$\Delta\quad\varphi \times \frac{180}{2\pi}$applied is represented in FIG. 5, making it possible to obtaindirectivity lobes with steep flanks so as to separate them better.

According to a characteristic of the invention, an RDT mode is appliedto the transmission antenna to obtain omnidirectional transmission onthe basis of a long pulse. FIG. 6 represents the radiation patternobtained according to sectors numbered from 1 to 12. The pulsetransmitted is divided into 6 juxtaposed slices of duration ΔT. Asrepresented in FIG. 7, each pulse slice provides a transmission along 2sectors. Each slice ΔT can correspond to a CW or wideband coded pulse,or both.

According to the invention, the reception antenna comprises a devicewith hydrophone trios which is integrated in a rigid manner into asingle linear antenna. Right/left channel forming is then performed asdescribed for example in French Patent No. 89 11749 filed on Sep. 8,1989 by the company Thomson-CSF, published on Mar. 15, 1991 under No. 2651 950 and granted on Apr. 17, 1992. Thus the ambiguity removal is thencarried out with a single transmitted pulse. According to an exemplaryembodiment, right/left discrimination is obtained in the 30°-150° and210°-330° bearing sectors.

FIGS. 8 and 9 respectively represent an assembly of modules of thereception antenna before sheathing, and such a reception module in anexploded view.

Each module contains 3 acceleration-insensitive hydrophones 9 positionedat the vertices of an equilateral triangle in a plane perpendicular tothe axis of the antenna (the third is hidden in the figure). Thesehydrophones are supported by a plate 90 in which is made a housing 91for installing a small cylindrical container containing the receptionelectronics. The plate is held in place by means of shoulders 92 made ontwo parts 93 and 94 forming the module together with a third part 95. Anannular piece 96 furnished with lugs ensures mutual torsionless holdingof the modules. The assembly is held in the sheath by the centeringpieces 80. A compact reception antenna with ambiguity removal is thusobtained making it possible to obtain directivity in the vertical plane.

According to an exemplary embodiment, the transmission and receptionantennas have a diameter equal to around 85 mm, the frequency band beingsituated around 1.5 Hertz, and the reception antenna is composed of 128modules, i.e. 3×128 reception channels, and directivity is obtained inthe vertical plane lying between 110° and 120°.

1. A towed low-frequency underwater detection system comprising: inseries on one and the same towline a linear transmission antennafollowed by a linear reception antenna with ambiguity removal whereinthe transmission antenna being in a substantially horizontal planecomprises a plurality of flextensional transducers, each transducerbeing housed in a cylindrical shell; and means for powering thesetransducers in such a way as to form transmission beams covering thewhole space, the linear transmission and the linear reception antennashaving a substantially the same diameter.
 2. The system as claimed inclaim 1, of which the linear reception antenna is formed of receptionmodules, wherein each of these modules comprises 3 hydrophones in aplane perpendicular to the axis of the antenna.
 3. The system as claimedin claim 1, wherein 3 distinct modes of transmission are used, onedirectional mode, a sectorial mode and a Rotational DirectionalTransmission (RDT) mode.
 4. The system as claimed in claim 3, whereinthe RDT mode is a double-beam mode.
 5. The system as claimed in claim 2,wherein 3 distinct modes of transmission are used, a directional mode asectorial mode and a RDT mode.
 6. The system as claimed in claim 5,wherein the RDT mode is a double-beam mode.
 7. The system as claimed inclaim 4, wherein a RDT transmission mode is used, an omni directional orselected wide sectorial transmission being obtained by means ofsuccessive wide and steep flank beams forming a long pulse.
 8. Thesystem as claimed in claim 7 wherein, in RDT mode, an omni directionaltransmission is obtained by means of the said successive wide and stepflank beams, the number of said successive beams being less than thenumber of the transducers of the transmission antenna.